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Here is a pretty typical "topic screen". Everything on this screen has to do with the concept, in tax law, of "Fair Market Value". On the topic screen, relationships to other different kinds of subjects are displayed in different ways. One kind of subject is called an "occurrence". In traditional Topic Map parlance, an "occurrence" is a bit of information, found in some larger document or corpus of documents, that is relevant to the subject. On these pages, occurrences are further subdivided into the corpora in which they appear. The first ones to be listed are the ones that are found in IRS Tax Publications, which is why you see the word "Publications" at the top of the middle column. Farther down the page, you may also find occurrences that are found in the IRS's FAQs, and in another IRS corpus called "Tele-Tax Topics". Other IRS corpora are gradually being added to Tax Map, by the way. The more they incorporate into Tax Map, the more they want to incorporate.

I want to talk more about occurrences, but first I want to direct your attention to the right-hand column, in which you'll find a list of related "topic screens" -- screens that are like this one, but that are all organized around other specific topics. The one at the top of the list is "Books: fair market value".

Now let's go back to occurrences. I've already said that occurrences are subjects, like any other subject, but I want to emphasize it heavily. A subject is a subject, regardless of whether it's an abstract concept, like "Fair Market Value", or a particular paragraph in a particular document, or, for that matter, Minnie Mouse's high-heeled shoes. The fact that different kinds of subjects are rendered differently on these topic pages obscures this, but there is a high level of abstraction at which all subjects are treated in exactly the same way by the Topic Maps paradigm.